Alternative Vicarious Gain Estimates for Sentinel-4 OLCI: Investigation of Atmosphere-typed Spectral Optical Thickness Corrections for the Processor Vicarious Calibration, From the Open Ocean to the Shore

Vicarious calibration aims at fitting TOA observations to estimates. The historical formulation of the gain G*ρ_TOA= ρ_path+T*ρ_w allows centering the model with the observations. Nevertheless, this formulation doesn’t allow discretising errors made either in the transmittance T or in the atmosphere reflectance ρ_path,. i.e. identifying the error sources.We propose here a vicarious calibration, both in shape and amplitude, of the estimated spectral optical thicknesses. We show that, in open ocean waters, using the GlobColor dataset complemented by data collected by MOBY, that the direct model used to calculate ρ_TOA underestimates the total optical thicknesses for some 2% at 400nm to 0.02% at 681nm. The new formulation of the vicarious adjustment involves a non-linear correction which adjusts in a better way than the historical formulation both the atmospheric components (ρ_path, T) and the water reflectances ρ_w.Discrepancies are also assessed in coastal areas, and it is possible to correct them using a vicarious calibration scheme which involves probability-based transitions between clear maritime and continental aerosol-loaded coastal atmospheres. By introducing continuous transitions between multiple modes, we maximize the fitness of ρ_w in the two cases. It may initiate the use of conditional gains for the current level 2 processors of optical EOs.An example will be shown for OLCI, with the provision of gains values, including their uncertainty estimates, and the improvement of ρ_w in absolute and relative terms.Keywords: Alternative vicarious calibration for OLCI, MODIS, VIIRS; Spectral optical thickness corrections; Atmosphere-typed mixture of gains Acknowledgments: This research is a follow-up of the work for the E.U. Copernicus Marine Service Information http://marine.copernicus.eu/ , and the Sentinel-3 Mission Performance Centre